1. Field of the Invention
The present invention relates to a container neck and closure assembly for use on pressurized containers such as carbonated beverage containers.
2. Description Of the Related Art
Current commercially mass-produced carbonated beverage containers use threads on the container and closure of the continuous, helical type. The threads comprise a single, substantially continuous thread portion on the container neck with a low thread pitch angle, typically less than 5.degree.. The low pitch angle is needed in order to ensure that the closure does not unscrew spontaneously under pressure from inside the container. The low pitch angle also provides the necessary leverage to achieve a gas-tight compressive seal between the closure and the container neck when the closure is tightened onto the container neck. The low pitch of the helical threads also means that the closure typically needs to be rotated through more than 360.degree. to disengage it completely from the container neck. Whilst this can be laborious, especially for elderly or child users, it also permits some gas venting to take place while the closure is being unscrewed, and thereby reduces the risk that the closure will blow off uncontrollably once unscrewing of the closure from the container neck has commenced. This gas venting is usually assisted by the provision of axial gas venting notches extending longitudinally through the helical threads.
Drawbacks of these low pitch helical threads include the laborious rotation required to remove and resecure the closure on the neck, excessive use of molding material to form the long helical threads, and unreliable separation of tamper-evident rings from the closure skirt due to the low pitch angle of the threads.
U.S. Pat. No. 5,135,124 describes a container closure assembly for a carbonated beverage container that incorporates a safety feature to prevent the closure blowing of off uncontrollably (missiling) as it is unscrewed from a container under high pressure. The closure assembly is provided with a complex double-bayonet thread arrangement to provide for gas venting at an intermediate position of the closure on the container neck. The bayonet thread arrangement can be difficult for infirm or very young users to assemble and disassemble successfully, since these operations involve sequential steps of pressing down and rotating the closure. Moreover, to achieve a pressure-tight seal, a strong axial sealing force must be applied by the user in the initial pressing-down step of securing the closure on the container neck. Furthermore, the bayonet-type threads are inherently less suitable for reliable operation of a tamper-evident ring that is frangibly attached to the closure skirt, but that is retained on the container neck after the assembly is opened for the first time.
The present applicant has described an improved pressure safety cap for carbonated beverage containers in International Patent Publication WO95/05322. This application describes container closure assemblies having substantially continuous threads defining a substantially continuous helical thread path, although the pitch of the helix can vary. The closure can be moved from a fully disengaged to a fully secured position on the container neck by rotation through 360.degree. or less. The threads on the neck or the closure are provided with mutually engageable elements to block or restrict rotation of the closure in an unscrewing direction beyond an intermediate position when the closure is under an axial pressure in a direction emerging from the container neck, the neck and closure being constructed and arranged to provide a vent for venting gas from the container neck at least when the closure is in the intermediate position. This pressure safety feature prevents the closure from blowing off uncontrollably once unscrewing of the closure from the container neck has started. It thus allows the use of shorter, more steeply pitched or multiple-start threads in the container and closure assembly, thereby rendering the assembly much more elderly- and child-friendly without sacrificing pressure safety.
WO97/21602 describes an improved version of the assemblies of WO95/05322 in which the thread on the container neck has a lower surface having a variable pitch, such that the pitch of thread is lower in a region near the bottom of the thread. This reduces the tendency of the closure to blow off when the container is sealed and pressurized. A further region of low pitch may be provided on the neck thread adjacent to the intermediate position where gas venting takes place. This reduces the tendency of the closure to override the blocking means at the intermediate position while gas venting is taking place.
GB-A-2288390 describes container closure assemblies for beverage containers. The closure cap screws onto the container neck in less that half a turn, with pins carried on the cap engaging between screw threads provided on the container neck. The threads are variably pitched to give a decreased final angle of pitch in order to reduce the likelihood that pressure exerted on the cap will cause the cap to back off the container neck. Slots may be provided on the underside of the threads to block unscrewing of the cap beyond an intermediate position until venting of pressure from inside the container has taken place.